The invention relates to a method and apparatus for operating an industrial furnace system wherein heat obtained from burning a fuel is supplied to a material to be treated and waste heat is generated.
It is known that numerous industrial processes are carried out in industrial furnace systems by supplying heat energy. For example, in the manufacture of expanded clay, cement, lime, fertilizers or the like, a certain raw material must be subjected to heat treatment in rotary kilns. Fired vertical kilns for calcining and roastings, oxidizing or sintering ores, and also sintering belts or fluidized bed furnaces are likewise known.
What all these processes have in common is that they have relatively high heat requirements which to date have been met by using high quality fuels, for example, oil, gas, coal or coke. In view of the scarcity of most of these fuels, the financial expenditure for the heat requirements in these industrial furnace systems is very high.
Relatively high amounts of waste heat are produced during operation of many industrial furnace systems, for example, by combustion machine surface radiation or through cooling devices used in the system which require large amounts of air to cool a treated material relatively quickly so that it can be further processed. The waste heat generated is in the form of hot air whose temperature (for example, between approximately 200.degree. and 400.degree. C.) is not high enough to make reuse in the same system appear worthwhile.
It is also known that the utilization of waste coming from municipal waste disposal installations, for example, sewage slurry, is problematic owing to high water content. Such waste, including, for example, sewage slurry with a relatively low moisture content, has been burnt in a fluidized bed furnace, and the combustion gases used for operating a compressor-turbine-unit (German published patent application No. 1943776). However, the problem of reusing the waste heat of the cooling device of an industrial furnace system did not occur, and with the exception of the relatively robust shovels of the turbine, the combustion gases from the fluidized bed furnace did not come into contact with any other material to be treated, as is the case in industrial furnace systems.
In effect, the combustion heat of the combustible substance contained in sewage slurry is relatively high, for example, approximately 8,000 kcal/kg of pure fuel. The combustion heat is only reduced by the high ash and water content in the sewage slurry. In the dry state the combustion heat of this material is still approximately 4,000 kcal/kg (approximately 50% to 60% of the combustion heat of a good sort of coal). It is only by the high water content of the sewage slurry which even after filtration lies between approximately 40% and 70%, more particularly, between approximately 50% and 60% water, that is usability is practically destroyed. For if filtered sewage slurry with 4,000 kcal/kg of combustion heat contains 70% water in the dry substance, the combustion heat drops to 30% owing to this circumstance alone for 1 kilogram of "fuel" contains only approximately 0.3 kilograms of combustible substance. Furthermore, the water must be evaporated during combustion, and in the event that water vapor is to be the evaporation product, a further 406 kcal/kg, based on the fuel used, are required. Consequently, a combustion heat of 794 kcal/kg is what remains in the end for the sewage slurry. However, since most thermal processes require high flame temperatures in order to attain the process temperatures and perform the processes in a short time, and furthermore, since the flame temperatures are a function of combustion heat and excess air, the aforesaid waste fuels have to date been unable to be used in practically all industrial furnace systems.